title: Evidence for the cation-pi interaction between Cu2+ and tryptophan.
abstract: The cation-pi interaction, a noncovalent interaction of electrostatic nature between a cation and an electron-rich pi system, is increasingly recognized as an important force that influences the structures and functions of molecules including proteins. Unlike other metal cations, the transition metal cation Cu2+ is not regarded to take part in a cation-pi interaction because Cu2+ tends to oxidize the pi electron system, in particular that of Trp, and to introduce covalency in the metal-pi electron interaction. This paper reports the first spectral evidence for the cation-pi interaction between Cu2+ and Trp. The Cu2+ ion bound to the amino N-terminal Cu2+/Ni2+ binding motif composed of three amino acid residues interacts with the indole ring of the fourth Trp residue in a noncovalent manner. The Cu2+-Trp interaction produces a distinct negative band at 223 nm in circular dichroism (CD), which disappears upon mutation or depletion of the Trp residue or upon replacement of the Cu2+ ion by Ni2+. In UV absorption, a pair of negative/positive intensity changes is generated at 222/231 nm by the Cu2+-Trp interaction, being consistent with the previous observations on the indole ring interacting with K+ or a cationic His imidazole ring. The negative CD band around 223 nm is characteristic of the Cu2+-Trp pair and may be useful as a marker of the Cu2+-Trp cation-pi interaction. Coordination of negatively charged ligands to Cu2+ is suggested to be important for the cation to be involved in a cation-pi interaction.
authors.affiliation: Graduate School of Pharmaceutical Sciences, Tohoku University, Aobayama, Sendai 980-8578, Japan.
authors.forename: Hanami
authors.initials: H
authors.lastname: Yorita
authors.forename: Kohei
authors.initials: K
authors.lastname: Otomo
authors.forename: Hirotsugu
authors.initials: H
authors.lastname: Hiramatsu
authors.forename: Akira
authors.initials: A
authors.lastname: Toyama
authors.forename: Takashi
authors.initials: T
authors.lastname: Miura
authors.forename: Hideo
authors.initials: H
authors.lastname: Takeuchi
mesh_terms: D002413:Cations, Divalent / Q000737:chemistry; D002942:Circular Dichroism; D003300:Copper / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D013053:Spectrophotometry; D014364:Tryptophan / Q000737:chemistry
chemical_list: D002413:Cations, Divalent; D003300:Copper; D014364:Tryptophan